1. Field of the Invention
The invention relates to ballpoint pens and to inks therefor.
2. Brief Description of Related Technology
In a conventional ballpoint pen, a reservoir column of thick ink bears onto the rear side of the writing ball. As the ball is rotated in its seat, the thick ink in contact with the ball is conveyed on the ball surface to be deposited on the paper or other substrate. As ink is withdrawn, the reservoir column of ink will flow down to maintain contact with the ball. This is vital since, if contact is lost and the ink will not flow on its own to re-establish contact with the ball, the pen becomes useless.
In the early days of ballpoint pens, the inks were oil-based. They suffered a number of disadvantages including bleed into paper, point bleed (oil separation) and messiness. The subsequent introduction of dye-based glycol inks, which contained resins to reduce smearing and improve temperature stability, gave a major improvement in performance. Currently, most ballpoint pen inks are based on organic solvents such as phenyl glycols, diethylene glycol ethylether, dipropylene glycol or benzyl alcohol, or mixtures of two or more thereof.
There have recently been developed a family of aqueous pigmented inks many of which display bright and attractive colours. In order to provide these inks in ballpoint pens, the pigments have been formulated as aqueous suspensions containing a thickener such as xanthan gum, carageenan gum, guar gum, locust bean gum or hydroxyethyl cellulose. These suspensions show shear thinning and can serve as inks in ballpoint pens. However, in use, a number of problems arise. For example, the aqueous suspension does not lubricate the ball in its seat and, as a result, severe wear can occur over a period of use and the pens do not write very smoothly.
One of the most important requirements of a ballpoint pen is that it should write very smoothly. The organic-based ballpoint inks generally (but not invariably) write much more smoothly than the recently developed aqueous-based inks, but even the organic based inks are not always as smooth in use as would be desired, and in addition they have the disadvantage of being somewhat sticky.
We have now found a way of improving the smoothness of organic-based ballpoint pen inks. Furthermore, we have found a way of providing pigmented inks whereby smooth writing can be obtained as well as a reduced ball/seat wear as compared with known aqueous-based ballpoint pen inks.